Stéphane Mottelet

Metabolic Flux Analysis in Isotope Labeling Experiments Using the Adjoint Approach

Comprehension of metabolic pathways is considerably enhanced by metabolic flux analysis (MFA-ILE) in isotope labeling experiments. The balance equations are given by hundreds of algebraic (stationary MFA) or ordinary differential equations (nonstationary MFA), and reducing the number of operations is therefore a crucial part of reducing the computation cost. The main bottleneck for deterministic algorithms is the computation of derivatives, particularly for nonstationary MFA. In this article, we explain how the overall identification process may be speeded up by using the adjoint approach to compute the gradient of the residual sum of squares. The proposed approach shows significant improvements in terms of complexity and computation time when it is compared with the usual (direct) approach. Numerical results are obtained for the central metabolic pathways of Escherichia coli and are validated against reference software in the stationary case. The methods and algorithms described in this paper are included in the sysmetab software package distributed under an Open Source license at http://forge.scilab.org/index.php/p/sysmetab/.

Modeling of an aerobic bioprocess based on gas exchange and dynamics: a novel approach

Monitoring of the biological degradation of a substrate by microorganisms is a key issue, especially in the soil bioremediation area. Respiration measurement is the easiest way to obtain online information on the biological activity. Nevertheless, it is indirectly related to substrate consumption and microbial growth. To be able to link these phenomena, a robust and descriptive model has been developed. Both biological and gas/liquid transfer dynamics must be taken into account to link the online measurement with the actual biological respiration. For that, experimental evolution of the respiratory ouotient (RQ) during a biodegradation has been compared against general biodegradation knowledge. To obtain a reliable model, practical and structural sensitivity analyses have been conducted. The model can describe the evolution of both online measurable and non-measurable states. It also gives a new definition of the apparent RQ, measured in the gas phase, compared to the actual biological RQ.

Smart Depth of Field Optimization Applied to a Robotised View Camera

The great flexibility of a view camera allows the acquisition of high quality images that would not be possible any other way. Bringing a given object into focus is however a long and tedious task, although the underlying optical laws are known. A fundamental parameter is the aperture of the lens entrance pupil because it directly affects the depth of field. The smaller the aperture, the larger the depth of field. However a too small aperture destroys the sharpness of the image because of diffraction on the pupil edges. Hence, the desired optimal configuration of the camera is such that the object is in focus with the greatest possible lens aperture. In this paper, we show that when the object is a convex polyhedron, an elegant solution to this problem can be found. It takes the form of a constrained optimization problem, for which theoretical and numerical results are given. The optimization algorithm has been implemented on the prototype of a robotised view camera.

Improving fairness between close Wi-Fi access points

We consider some Wi-Fi access points (AP) communicating with some stations while being in the carrier sense area of neighbor AP. This corresponds to usual scenarios in local area networks, set-top boxes in houses, intelligent transportation systems, etc. First, using the network simulator ns-3, we observe interesting phenomena, depending on the number of pairs and its parity, confirming previous experiments and models of the literature. Short chains give high disparity in throughput. Moreover, adding a pair (i.e., an AP) can drastically change the throughput of a far ones. We also point out a notable asymptotic behavior. We show that the same phenomena appear while varying the number of stations per AP, the sending rate or the chain geometry. Second, we provide a new analytic model by considering the probability for a station to send data while its neighbors are waiting. This powerful model leads to a non-linear system of equations matching very well the Ns-3 simulations. It is scalable and permits to study accurately both short and very large chains. In particular, we show interesting properties related to the fairness. Third, we apply our results to some practical cases. Thanks to our model, we are able to compute the optimal settings of the stations for ensuring the fairness in the scenarios listed above.

Controllability and stabilization of a moving water tank system considering fluid-structure interaction

This paper deals with the modeling and the mathematical analysis of problems involving a rectangular container installed on a cart. The overall system is controlled via a longitudinal force in order to move the cart from one location to another, and the key problem is the suppression of sloshing during transportation. We firstly make a precise mathematical analysis of the model in order to obtain a system under an adequate form, in order to study some controllability and stabilization problems with standard techniques. Note that in this paper, we want to control and stabilize simultaneously the fluid and the cart. We show that the system is not approximately controllable in finite time, but only on [0, infin). We also study the stabilization of the system with two different static feedbacks. The first feedback uses the observation of the cart speed only. The second feedback uses the cart speed and a measurement of the elevation of the free surface of the fluid in the container. We show that strong stability holds in both cases, although the coupled system is not dissipative for the second type of feedback